Cast irons pitting corrosion

Cast iron corrodes because of the exposure of graphite content of cast iron (graphitic corrosion), which is cathodic to both low alloy and mild steels. The trim of a valve must be cathodic to the valve body to avoid pitting attack. Thus in aggressive media valve bodies of steel are preferred to cast iron bodies. Steel bolts and nuts coupled to underground mild steel pipes or a weld rod used for steel plates on the hull of a ship should always be of a low nickel, low chromium steel, or from a similar composition to that of the steel pipe (8). 

An acceptable life for undergound pipelines can sometimes be realized through the use of corrosion-resistant piping. Copper, aluminum, and stainless steel piping are sometimes used for this purpose. All three alloys can have greater corrosion resistance than carbon steel or cast iron. However, they are not immune to corrosion and are often more susceptible to localized corrosion such as pitting, crevice corrosion, and SCC than carbon steel or cast iron. Further, corrosion protection in the form of coatings and cathodic protection are frequently used. 

When the layer of graphite and corrosion products is impervious to the solution, corrosion wdl cease or slow down. If the layer is porous, corrosion will progress by galvanic behavior between graphite and iron. The rate of this attack will be approximately that for the maximum penetration of steel by pitting. The layer of graphite formed may also be effective in reducing the g vanic action between cast iron and more noble alloys such as bronze used for valve trim and impellers in pumps. 

Local corrosion or pitting is more important for practical purposes than the rate of general corrosion, and may proceed 10 times or so more rapidly than this. Inasmuch as certain types of cast iron are liable to suffer graphitic corrosion, whereas steel does not, steel might theoretically be expected to show to some advantage when used for buried pipelines. In practice, however, a cast-iron pipe has to be of stouter wall than a steel pipe for equal strength, and it is doubtful whether any distinction between the rust resistance of the two materials in the soil is justified. 

Very rapid and highly localised pitting is sometimes observed on components exposed to very turbulent flow conditions leading to cavitation in the stream. In general, these conditions appear to induce corrosion rather than erosion on cast iron surfaces, in contradistinction to what usually happens with other metals, apparently because the erosive component of the liquid flow scours away corrosion-stifling films and allows the development of very active electrochemical cells on the exposed metal surfaces . 

A characteristic of the corrosion on buried ferrous metals is that the attack is usually mostly in the form of pitting, especially with the cast irons. This raises a problem in measuring the extent of corrosion in burial trials. Usually both the weight loss, measuring the average loss of section, and the deepest pit, measuring the maximum loss of section, are reported. For assessing the severity of the attack on buried pipes, the second parameter is clearly the most important. 

Cast Iron. The iron phase in cast iron is readily attacked by sea water, as is the case for mild steel. If the layer of graphite left with the corrosion product is dense and compact, further corrosion tends to be stifled. If the layer is porous, corrosion may be accelerated by the galvanic action between the graphite and the iron beneath. The attack then approaches a rate similar to that found for the pitting of mild steel. 

The corrosion rate of ductile iron pipes in soils was in the range 0.62-2.5 mm/yr with an average of 1.11 mm/yr which is twice the rate of cast iron pipe. Failure of ductile iron pipe was solely due to localized pitting corrosion leading to pipe wall perforation. 

Phosphates and silicate corrosion inhibitors have been used with or without pH control, to reduce the metal release and to prolong the service life of distribution systems or domestic installations. When the concentration is limited, the inhibitors may not avert localized corrosion such as pitting or the corrosion of galvanized steel, steel, cast iron, copper, or lead, sufficiently to extend the life of the system beyond 75-100 years. Corrosion inhibitors are useful when concerns about water quality deterioration have to be resolved. Unfortunately, there is no simple solution for balancing water quality, health risks, system reliability, and environmental impact. 

Figures 5.72 and 5.73 show the corrosive attack on samples of cast iron pipe and ductile iron pipe buried under the soil for 20 and 9 years, respectively. The large hole in cast iron pipe (Fig. 5.72) and the corrosion pit and perforation in ductile iron pipe (Fig. 5.73) show the severity of soil corrosion. It is suggested that cathodic protection can reduce the extent of corrosion of iron pipes.

Cast iron is initially anodic to low-alloy steels and not far different in potential from mild steel. As cast iron corrodes, however, especially if graphitic corrosion takes place, exposed graphite on the surface shifts the potential in the noble direction. After some time, therefore, depending on the environment, cast iron may achieve a potential cathodic to both low-alloy steels and mild steel. This behavior is important in designing valves, for example. The trim of valve seats must maintain dimensional accuracy and be free of pits consequently, the trim must always be chosen cathodic to the valve body making up the major internal area of the valve. For this reason, valve bodies of steel are often preferred to cast iron for aqueous media of high electrical conductivity. 

A metal pipeline was unlikely to be satisfactory in this application. Cast iron would be expected to last perhaps 20-30 years, whereas the design life was to be 50 years. It would be subject to internal attack from the effluent and external attack from the sea water. Protection against this type of corrosion would be difficult. Similar objections apply to use of mild steel, where again predictable protection would be difficult to achieve mild steel would also suffer rapid internal corrosion. A suitable grade of stainless steel could probably be selected to resist internal attack by the effluent but the problem of external corrosion would remain. Stainless steel is subject to corrosion pitting in sea water, especially when the oxygen content is low. In quiet water the rate of pitting can be 6.9 mm per year. 

Some austenitic cast iron types may show a level of sensitivity to stress corrosion cracking in seawater and salt solutions at higher temperatures [96]. At high nickel contents of around 35%, pitting corrosion increases [97]. 

The standard steels of the type SAE 316 (DIN-Mat. No. 1.4401, X5CrNiMol7-12-2) are not suitable for seawater-exposed pipes and fail as a result of pitting and crevice corrosion [155, 156]. The sensitivity to pitting corrosion of these standard steels can be further increased by deposits of maritime bacterial films [157]. Despite these facts, these steels are frequently used as materials for pump parts and have worked well as such because they are cathodically protected by contact with other parts made of less noble materials, e.g. pump casing made of cast iron [130]. [158] reports on tests of the cavitation behaviour of the pump materials GX5CrNiMol9-ll-2 (DIN-Mat. No. 1.4408) in 3% NaQ solution. 

The corrosion rates of these materials in almost neutral waste waters are chiefly determined by the concentration of oxygen and by its transport to the surface of the material. However, in addition to uniform surface attack, which must be taken into account by increasing the wall thickness, there may be increased local attack in the form of shallow pit corrosion and pitting corrosion due to the formation of aeration cells. The use of unalloyed and low-alloy steels as well as cast iron and cast steel is generally not recommend if there are no additional corrosion protection measures, e.g. with coatings, linings, or cathodic protection. 

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